The present invention relates to improved substrates for printed circuit boards and to printed circuit boards for high-frequency applications such as microwave and milliwave applications.
In Japanese Patent Application No. 59-72,472, the applicant has proposed a substrate for a printed circuit board for high-frequency application which is characterized by small coefficients of thermal deformation, such as thermal expansion or shrinkage. This substrate is provided with an insulating layer which comprises a porous polytetrafluoroethylene resin element affixed to a polytetrafluoroethylene layer. It is known from Japanese Patent Publication (Kokai) No. 60-225,750 that electrical characteristics of substrates for printed circuit boards can be improved by arranging expanded porous polytetrafluoroethylene resin layers along conductive circuits. Such substrates for printed circuit boards, however, had conductive layers made of copper on both sides of a dielectric substrate, and, when they were subjected to bending, it became impossible to relieve stress caused by bending of the printed circuit board itself. As a result, cracks appeared in the conductive cooper layer.
It is an object of the present invention to eliminate the above disadvantages and to provide a substrate for a printed circuit board, with conductive layers on both sides of a dielectric body, in which, even under bending conditions of the printed circuit board substrate, it is possible to relieve the stress caused by bending and to prevent formation of cracks in the conductive layers as well.